Needs.....more
This is great, but while the number of enemies makes for a tougher challenge, it IS too much for one turret to handle. Maybe instead of bonus drops, you just switch turret types after buying them, granted, it's been done before but, there's no problem in going with what works. Another option would be if the bonuses accumulated, much like your "nuke" meter, then each shot would take a portion of energy to fire. Lastly, more bonus types would be a great addition; Filling the "nuke" meter, auto-firing turrets, that kind of thing.